linux-kernel - Re: [PATCH] /sched/core: Fix Unixbench spawn test regression

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Message-ID: <51d24e59-247d-4b66-9ddb-4b9f0f53497b@arm.com>
Date: Wed, 12 Mar 2025 15:41:40 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Juri Lelli <juri.lelli@...hat.com>, Steven Rostedt <rostedt@...dmis.org>,
 Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
 Valentin Schneider <vschneid@...hat.com>, Hagar Hemdan
 <hagarhem@...zon.com>, linux-kernel@...r.kernel.org, wuchi.zero@...il.com
Subject: Re: [PATCH] /sched/core: Fix Unixbench spawn test regression

On 11/03/2025 17:35, Vincent Guittot wrote:
> On Mon, 10 Mar 2025 at 16:29, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>
>> On 10/03/2025 14:59, Vincent Guittot wrote:
>>> On Thu, 6 Mar 2025 at 17:26, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>>>
>>>> Hagar reported a 30% drop in UnixBench spawn test with commit
>>>> eff6c8ce8d4d ("sched/core: Reduce cost of sched_move_task when config
>>>> autogroup") on a m6g.xlarge AWS EC2 instance with 4 vCPUs and 16 GiB RAM
>>>> (aarch64) (single level MC sched domain) [1].
>>>>
>>>> There is an early bail from sched_move_task() if p->sched_task_group is
>>>> equal to p's 'cpu cgroup' (sched_get_task_group()). E.g. both are
>>>> pointing to taskgroup '/user.slice/user-1000.slice/session-1.scope'
>>>> (Ubuntu '22.04.5 LTS').
>>>
>>> Isn't this same use case that has been used by commit eff6c8ce8d4d to
>>> show the benefit of adding the test if ((group ==
>>> tsk->sched_task_group) ?
>>> Adding Wuchi who added the condition
>>
>> IMHO, UnixBench spawn reports a performance number according to how many
>> tasks could be spawned whereas, IIUC, commit eff6c8ce8d4d was reporting
>> the time spend in sched_move_task().
> 
> But does not your patch revert the benefits shown in the figures of
> commit eff6c8ce8d4d ? It skipped sched_move task in do_exit autogroup
> and you adds it back

Yeah, we do need the PELT update in sched_change_group()
(task_change_group_fair()) in the do_exit() path to get the 30% score
back in 'UnixBench spawn'. Even that means we need more time due to this
in sched_move_task().

I retested this and it turns out that 'group == tsk->sched_task_group'
is only true when sched_move_task() is called from exit.

So to get the score back for 'UnixBench spawn' we should rather revert
commit eff6c8ce8d4d.

The analysis in my patch still holds though.

If you guys agree I can send the revert with my analysis in the
patch-header.